Ultrafiltration of oily water under different conditions, considering critical and limiting flux

It is over a decade since the concept of critical fl ux was introduced. From that time intensive research has been done to fi nd the functionality and dependency of critical fl ux for different feeds and membranes. The studies focussed on critical fl ux in the fi eld of oily water fi ltration are scarce, however, there are numerous membrane units which have been applied to treat wastewater contaminated with oil. It might be due to the fact that in this kind of fi ltration the oil drops form a gel layer on the surface of the membranes and this gel layer formation is not the case with feeds where the concept of critical fl ux has been studied (suspended solids forming a cake layer). In membrane fi ltration of oily water, knowledge on the existence of critical fl ux and the infl uence of different parameters on it may enhance a better operation of the membrane units. In order to fi nd the critical fl ux, a set of experiments was carried out in constant pressure mode using varying pressures from 0.5 to 4 bars. A cycling pressure approach (increasing and decreasing pressure) was applied to fi nd the possible critical fl ux in a system consisting of emulsifi ed oil and salts. It was found that a weak form of critical fl ux in some cases might exist close to the lowest pressure. No strong form of critical fl ux was observed from comparison between pure water and permeate fl ux since the permeate fl ux differed signifi cantly from the initial pure water fl ux even at the lowest pressure. The existence of a limiting fl ux and its dependency on operating conditions was proven in some cases. The limiting fl ux was established under different operating conditions, including oil concentration, pH, temperature, salts and fl ow velocity. The results showed that this fl ux was signifi cantly infl uenced by variations in fl ow velocity and temperature.